Most Irish and UK businesses running Microsoft 365 are paying for a third-party antivirus tool that duplicates — and in several areas underperforms — the security platform already included in their Microsoft 365 licence.

Microsoft Defender for Business is not the basic Windows Defender that shipped with Windows 10 in 2016. It has been rebuilt, significantly expanded, and as of 2026 it is a serious enterprise-grade security platform that competes directly with Sophos, ESET, and Bitdefender on endpoint protection — and surpasses all three on Microsoft 365-specific threat detection.

This post covers exactly what Defender for Business includes, how it compares to the leading third-party tools, what it catches that traditional antivirus misses, and what still needs to be configured correctly before it can be considered a complete security solution.

91%
of cyberattacks start with a phishing email — which traditional AV cannot block at the network layer
£0
additional licence cost for Defender for Business on Microsoft 365 Business Premium
6x
faster threat detection with AI-powered behavioural analysis vs signature-based AV

What Microsoft Defender for Business Actually Includes

The confusion in this space comes from the name. “Microsoft Defender” has referred to several different products over the years — the basic antivirus built into Windows, the consumer security suite, and now Defender for Business, which is the enterprise endpoint protection platform included in Microsoft 365 Business Premium and available as a standalone add-on for Business Basic and Standard.

Defender for Business is not a basic antivirus scanner. Here is what it actually includes in 2026.

Next-Generation Antivirus

AI-powered malware detection using behavioural analysis and cloud-based threat intelligence — not just signature-based scanning. Detects zero-day threats and fileless malware that traditional signature AV cannot catch.

Endpoint Detection and Response (EDR)

Continuous monitoring of all endpoint activity. When a threat is detected, Defender captures a full timeline of what happened — which files were touched, which processes ran, which network connections were made — enabling complete incident investigation.

Attack Surface Reduction Rules

Policies that block specific attack vectors before malware can execute — preventing Office macros from running code, blocking executable files from email attachments, and stopping credential theft from browser memory.

Automated Investigation and Remediation

When a threat is detected, Defender automatically investigates it, determines the scope, and remediates — quarantining files, reverting changes, and isolating compromised devices — without requiring manual IT intervention for every alert.

Defender for Microsoft 365

Email-specific threat protection — Safe Links rewrites and checks URLs in emails at click time, Safe Attachments detonates email attachments in a sandboxed environment before delivery, and anti-phishing policies detect impersonation attempts.

Microsoft Secure Score

A continuously updated security posture score that identifies specific configuration gaps and prioritises remediation actions across your entire Microsoft 365 environment — not just endpoints.

Defender vs Sophos, ESET and Bitdefender: The Honest Comparison

Third-party antivirus tools are not bad products. Sophos Intercept X, ESET Protect, and Bitdefender GravityZone are all capable endpoint protection platforms. The question for a business already on Microsoft 365 is whether the additional cost and management overhead of a separate tool is justified given what Defender already provides.

Capability Defender for Business Sophos Intercept X ESET Protect Bitdefender GravityZone
Next-gen AV Yes Yes Yes Yes
EDR Yes Yes Paid add-on Higher tier
Email threat protection Yes — native M365 Gateway only Gateway only Gateway only
SharePoint / Teams / OneDrive scanning Yes — native No No No
Attack surface reduction rules Yes Partial Partial Partial
Automated investigation & remediation Yes Yes Limited Yes
Conditional access integration Native — Entra ID Via connector Via connector Via connector
Additional licence cost (M365 Business Premium) Included €8–15/user/month €5–10/user/month €5–12/user/month

What Defender Catches That Traditional Antivirus Misses

The biggest gap between Defender and traditional antivirus tools is not in malware detection — all the platforms above perform comparably on standard malware in independent testing. The gap is in Microsoft 365-specific threat vectors that traditional AV tools simply cannot see.

Business Email Compromise

BEC attacks do not use malware. An attacker compromises an email account and sends payment instructions or data requests from a legitimate email address. Traditional AV cannot detect this — there is no file to scan. Defender for Microsoft 365’s anti-phishing policies use AI to detect impersonation patterns, unusual sending behaviour, and spoofed display names that indicate a compromised account, even when no malicious payload is present.

Malicious Links in Teams and SharePoint

Traditional antivirus scans files on endpoints. It cannot scan a URL shared in a Teams message or a link embedded in a SharePoint document. Defender’s Safe Links policy rewrites and checks every URL in Teams messages, SharePoint, and OneDrive at click time — blocking access to malicious destinations even when the link was safe when it was originally shared.

Fileless Malware

Fileless attacks execute entirely in memory — no file is written to disk, so there is nothing for signature-based scanning to find. Defender’s behavioural analysis monitors process activity, memory injection, and PowerShell execution patterns in real time, detecting fileless attacks by their behaviour rather than their file signature.

Lateral Movement After Initial Compromise

When an attacker gains access to one endpoint, they typically move laterally to other systems before executing their main objective. Defender’s EDR capability maps the full attack chain — from the initial compromised account through to every system and file touched — enabling complete incident response rather than just blocking the initial threat.

The businesses most exposed to Microsoft 365-specific attacks are those running a third-party AV that protects the endpoint but leaves the email layer, Teams, SharePoint, and OneDrive completely unmonitored. Defender covers the entire Microsoft 365 surface. No third-party tool does.

The Cost Argument

For a 25-person business on Microsoft 365 Business Premium paying €8 per user per month for Sophos, the annual security spend on antivirus alone is €2,400 — on top of the Microsoft 365 licence that already includes Defender for Business.

Moving to Defender does not mean reducing security capability. In most cases for Microsoft 365 businesses, it means improving it — specifically in email threat protection, Microsoft 365 content scanning, and the integrated security posture visibility that Secure Score provides. The cost savings are a byproduct of using the right tool for the environment rather than layering a generic endpoint tool on top of a Microsoft 365-specific threat landscape.

Is your Microsoft 365 security properly configured?

Mata365 provides a free Microsoft 365 security assessment — reviewing your current Defender configuration, identifying gaps, and giving you a prioritised plan to close them.

Explore M365 Security Services

What Still Needs Configuring

Defender for Business does not protect anything by default. Like every Microsoft 365 security feature, it ships in a permissive state and requires deliberate configuration before it is doing what it is capable of. Most businesses that believe Defender is active have it switched on but not configured — which is meaningfully different.

  • Attack Surface Reduction rules must be enabled and moved from Audit mode to Block mode — in Audit mode, Defender logs what it would have blocked but does not actually block anything.
  • Safe Links policies must be configured to rewrite links in email, Teams, and Office apps, and to block users from clicking through to unsafe URLs even after an override warning.
  • Safe Attachments policies must be set to Dynamic Delivery mode — this detonates attachments in a sandbox before delivery rather than simply scanning them at rest.
  • Anti-phishing policies must be configured with impersonation protection for key executives and domains, and with spoof intelligence rules that detect display name impersonation.
  • Conditional access policies must be configured to require compliant devices — Defender marks devices as compliant or non-compliant, but this signal is only enforced if conditional access policies are in place to act on it.
  • Automated investigation and remediation must be set to Full automation rather than the default Semi-automated mode — Semi-automated requires manual approval for every remediation action, which negates most of the benefit.
  • Alert and incident notifications must be configured so that someone is actually notified when Defender detects a threat. By default, alerts go to the Microsoft 365 security portal — which nobody is checking unless they know it exists.

Where to Start

If your business is running Microsoft 365 Business Premium and a separate third-party antivirus, the right starting point is a Defender configuration review — checking what is active, what is in audit mode, and what has never been touched — before deciding whether to consolidate onto Defender or maintain both.

For most Irish and UK SMEs on Microsoft 365, a properly configured Defender for Business deployment provides better Microsoft 365-specific protection than a third-party tool, at no additional cost, with a single unified management interface. The barrier is not the technology — it is the configuration work that was skipped during the original Microsoft 365 setup.

Mata365’s Microsoft 365 security service covers the full Defender configuration — Safe Links, Safe Attachments, anti-phishing, ASR rules, conditional access integration, and alert routing — for Irish and UK businesses that want their Microsoft 365 security actually working.

Free Microsoft 365 Security Assessment

Mata365 reviews your current Defender configuration, identifies what is active versus what is in audit mode or disabled, and gives you a prioritised plan — no commitment required.

Book Free Assessment Explore Security Services